Seismic survey vessels

ABSTRACT

A seismic survey vessel having a displacement monohull is provided with a streamer deck which width increases progressively from amidships towards the stern of the vessel, on both sides of the centerline of the vessel. This increased width of the streamer deck, reaching 37.5 meters at the stern, facilitates the deployment and towing of particularly wide streamer arrays, containing as many as sixteen streamers. It also provides space for the installation of novel streamer handling systems, one for each streamer, across the width of the rear of the streamer deck.

BACKGROUND OF THE INVENTION

This invention relates to seismic survey vessels, and is moreparticularly concerned with seismic survey vessels suitable for use inperforming 3D marine seismic surveys covering large areas.

In order to perform a 3D marine seismic survey, a plurality of seismicstreamers, each typically several thousand meters long and containingarrays of hydrophones and associated electronic equipment distributedalong its length, are towed at about 5 knots behind a seismic surveyvessel, which also tows one or more seismic sources, typically air guns.Acoustic signals produced by the seismic sources are directed downthrough the water into the earth beneath, where they are reflected fromthe various strata. The reflected signals are received by thehydrophones in the streamers, digitised and then transmitted to theseismic survey vessel, where they are recorded and at least partiallyprocessed with the ultimate aim of building up a representation of theearth strata in the area being surveyed.

At the present time, a typical streamer array used by the Applicantcomprises a 700 meter wide array of eight evenly spaced streamers, eachabout 4000 meters long. The streamers are towed by their lead-ins, iethe armoured electrical cables that convey electrical power, control anddata signals between the vessel and the streamers, as described in theApplicant's U.S. Pat. No. 4,798,156, and their spread is controlled andmaintained by MONOWING deflectors of the kind described in theApplicant's U.S. Pat. No. 5,357,892.

An array of this relatively large size allows 3D seismic surveys oflarge areas to be performed very efficiently. However, because the costof performing such surveys is so high, there is continuous pressure toimprove efficiency still further.

SUMMARY OF THE INVENTION

One way of achieving this is to use even wider streamer arrays,containing even.

According to the present invention, therefore, there is provided aseismic survey vessel having a displacement monohull, and powered drummeans and cooperating guide means for deploying and recovering seismicstreamers over the stern of the vessel via a streamer deck disposed at asubstantial height above the waterline of the vessel, wherein the drummeans are mounted substantially amidships in the vessel, the streamerdeck at the stern of the vessel projects laterally outwardly from thehull of the vessel on both sides of the centre line of the vessel and iswider than the remainder of the vessel, and the guide means includesguide devices distributed across substantially the whole width of thestreamer deck to facilitate the lateral spreading of the streamers.

In a preferred embodiment of the invention, the streamer deck of thevessel is an upper deck (although not necessarily the uppermost deck),at a height such that when the vessel is normally docked, the deck iswell above the dock.

Preferably the width of the streamer deck decreases progressively fromthe stern of the vessel until it becomes substantially equal to thewidth of the vessel amidships.

Advantageously, at least parts of the drum means are mounted in thevessel at a level lower than the streamer deck.

The vessel preferably includes a further deck below the streamer deck atthe stern of the vessel, said further deck being provided with means fordeploying and recovering seismic source means over the stern of thevessel.

In a particularly advantageous implementation of the invention, theguide means comprises a respective streamer handling apparatusassociated with each of a plurality of the streamers, each suchapparatus comprising:

a beam;

means for mounting the beam above normal head height above the streamerdeck of the vessel, such that the beam extends generally longitudinallyof the vessel, the mounting means including a pivotal connection to thesternmost end of the beam permitting said end to pivot about ahorizontal axis extending generally transversely of the beam, andsupport means for lowering and raising the other end of the beam towardsand away from the streamer deck; and

a respective one of said guide devices, each such guide devicecomprising pulley means secured to and movable along the beam.

Advantageously, the mounting means is secured to the underside of afurther deck of the vessel, above the streamer deck.

Conveniently, the pulley means comprises carriage means movable alongthe beam, a pulley device, and means for suspending the pulley devicebeneath the carriage means.

Preferably, the suspension means comprises a collar which is rotatablysupported on the carriage means and which is rotatable about thelongitudinal axis of the beam.

Additionally, the pulley device is preferably pivotable about ahorizontal axis which extends generally transversely of the beam.

The carriage means preferably includes at least one hydraulic motorarranged to move it in both directions along the beam, and isadvantageously provided with hoisting means at its sternmost end.

Conveniently, the support means comprises a telescopic member which ispivotally connected at one end to the mounting means and at the otherend to said other end of the beam, and is hydraulically operated.

The apparatus may further comprise downwardly projecting guidance meanswhich is positioned at said other end of the beam and which engages andpushes down the streamer when said other end of the beam is lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings, of which:

FIG. 1 is a plan view of a deep marine seismic survey vessel inaccordance with the present invention, substantially at the level of itswaterline;

FIG. 2 is a plan view of the streamer deck of the vessel of FIG. 1;

FIG. 3 is a stern view of the vessel of FIGS. 1 and 2;

FIG. 4 is a side view of streamer handling apparatus incorporated in thevessel of FIGS. 1 to 3; and

FIGS. 5 and 6 are top and end views respectively of the streamerhandling apparatus of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The deep marine seismic survey vessel shown in the drawings is indicatedgenerally at 10, and has a displacement monohull 12 which is typicallyjust over 80 meters long at the waterline (and just under 90 meters longoverall), with a beam of about 24 meters. The maximum displacement ofthe vessel 10 is typically about 7500 tonnes.

As can be seen in FIG. 1, the shape of the hull 12 at the waterline isfairly conventional, in that the width of the hull increases smoothlyand progressively from the bow 14 to a centre section 16 ofsubstantially uniform width, and then progressively decreases, but to amuch lesser extent, from the centre section to a wide cut-off stern 18.Below the waterline, the taper towards the stern 18 increasesprogressively with depth.

The vessel 10 has most if not all of the major features common to avessel of its type and size, eg multiple diesel engines, bow thrusters,electric generators, accommodation for 60 to 70 persons, a helideck,winches/derricks, emergency equipment, etc. However, since thesefeatures can take any of several well known and conventional forms, andare not germane to the present invention, they will not be described indetail for the sake of simplicity.

Additionally, the vessel 10 is arranged in accordance with the inventionfor performing deep marine seismic surveys, as will now be described.

Thus as can be seen in FIG. 2, the vessel 10 has a streamer deck 20which extends aft from the centre section 16, increasing linearly inwidth as it does so from about 24 meters at the centre section to about37.5 meters at the stern 18, the increase in width taking placesymmetrically on each side of the fore-and-aft centre line of the vessel10. The overall length of the streamer deck is about 36 meters, whilethe height of the streamer deck 20 above the waterline of the vessel 10is typically about 9 meters.

Within the centre section 16 of the vessel 10, sixteen large poweredstreamer drums 22 are distributed across the width of the vessel, eachdrum having its axis extending substantially horizontally andtransversely of the vessel 10 and being capable of storing a respectivestreamer up to 6000 meters long, together with its lead-in. The outerfour drums 22 on each side of the centre line of the vessel 10 aremounted at least partly below the streamer deck 20, while the innereight drums are mounted on the streamer deck itself, the drums beingpartly interleaved to fit them in the available space. The lower drums22 overspool, while the upper ones underspool.

At the centre of the stern 18 of the vessel 10, beneath the streamerdeck 20, is a deck 24 known as the gun deck. From the gun deck 24,seismic sources 26, typically multiple airgun seismic sources of thekind described in the Applicant's U.S. Pat. No. 4,686,660, are deployed,typically using a handling system of the kind described in Applicant'sU.S. Pat. No. 5,488,920.

In use, the streamers, indicated rather diagrammatically at 28, eachpass over a respective guide 30 and then along the length of thestreamer deck towards the stern 18 of the vessel 10. As they pass overthe streamer deck 20, the streamers 28 pass via respective tensionrelieving devices 32, and at the stern 28 they each pass over arespective further guide, this time in the form of a spooling block 34with its axis extending substantially vertically. To save space, thespooling blocks 34 can be of the type described and claimed in theApplicant's PCT Patent Application No PCT/IB97/00156 (WO 97/29302). Eachspooling block 34 forms part of a respective streamer handlingapparatus, which will be described in more detail hereinafter withreference to FIGS. 4 to 6.

Once deployed over the stern 18 of vessel 10, the twelve streamers 28,towed at about 5 knots by the vessel, are formed into an array typicallyup to 1100 meters wide, using Applicant's MONOWING deflectors asdescribed hereinbefore. This wide streamer array is then used, inconjunction with the seismic sources 26, to perform a 3D marine seismicsurvey as described earlier.

The vessel 10 has a number of advantages over prior art seismic surveyvessels of comparable size. For example, the fact that the vessel 10 hasa substantially conventional displacement monohull means that it is notmuch more expensive to build than the prior art vessels. Further, thepowered drums 22 with the streamers 28 wound on them are extremelyheavy, so their location amidships, as low as conveniently possible,contributes to the stability and seaworthiness of the vessel 10.

Another major advantage of the vessel 10 lies in the substantiallyincreased width of the streamer deck 20 at the stern 18 of the vessel.This facilitates the formation of the extra wide streamer array, interalia by reducing the angles through which the lead-ins of the outerstreamers 32 in particular are required to bend at the stern of thevessel to form the array. This latter feature reduces the stresses towhich the lead-ins and the streamers 32 are subjected, so reducing thepossibility of breakage. The increased width also provides more space atthe stern 18 for working with the increased number of streamers 28 usedto form the extra wide array. But because the streamer deck 20 is 9meters above the waterline of the vessel 10, it does not significantlyinterfere with the docking of the vessel, since the deck will normallybe well above the dock and will therefore merely project over it.

The extra space across the width of the streamer deck 20 at the stern 18of the vessel 10 also creates sufficient room for the installation ofnew and improved streamer handling apparatus, one for each streamer:this new and improved streamer handling apparatus is shown in FIGS. 4 to6.

The streamer handling apparatus of FIGS. 4 to 6 is indicated generallyat 40, and comprises an elongate beam 42 which extends generallylongitudinally of the vessel 10. The beam 42 is pivotally mounted at itsend 44 nearer the stern 18 of the vessel to mounting structure 46, whichis firmly secured to the underside 48 of the deck 50 of the vesselimmediately above the streamer deck 20: the deck 50 is, in fact, theupper stern deck of the vessel.

The beams 42 of adjacent streamer handling apparatuses 40 divergeslightly, or fan out, towards the stern 18 of the vessel, to follow theincreasing width of the stern.

As will become apparent hereinafter, the beam 42 normally extendsparallel to the streamer deck 20 and the upper deck 50, ie substantiallyhorizontally, at a height of nearly 4 meters above the former. To thisend, the other end 52 of the beam 42 is pivotally connected to one end54 of a hydraulically operated extendable telescopic arm 56, whose otherend 58 is pivotally connected to the underside 48 of the upper deck 50.In its unextended state, the telescopic arm 56 securely holds the beamin its normal, horizontal, position, while extension of the telescopicarm 56 lowers the end 52 of the beam 42 to a height of about 1 meterabove the streamer deck 20, ie to about waist height. Both the raisedand lowered positions of the beam 42 are shown in FIG. 4

A curved guide 59, effectively a 90° segment of a pulley wheel, ispivotally secured to the joined ends 52, 54 of the beam 42 and thetelescopic arm 56 respectively.

Mounted on the beam 42 so as to be movable in both directions along itis a carriage 60. Motive power for the carriage 60 is provided byhydraulic motors 62, which typically move the carriage, on suitablypositioned wheels (not shown), along the beam via a rack and piniondrive arrangement (not shown). Pressurised hydraulic fluid for themotors 62 is supplied via a long flexible pressure hose 63, whichunfolds as the carriage 60 moves along the beam 42.

The carriage 60 rotatably supports a collar 64, which coaxiallysurrounds both the carriage and the beam 42 and is rotatable about thelongitudinal axis of the beam. The collar 64 supports in turn thespooling block 34, which is effectively a particularly compactequivalent of a large radius pulley or fairlead. The spooling block 34is pivotally suspended beneath the collar 64, so as to pivot about anaxis extending generally transversely of the beam 42.

The end 66 of the carriage 60 nearer the stern end 44 of the beam 42 isprovided with a winch 68.

In normal use, ie when the streamer 28 handled by the apparatus 40 isdeployed and being towed, the beam 42 is held in its raised horizontalposition, and the carriage 60 is locked at the stern end 44 of the beam.The streamer 28 passes over the spooling block 34, which is free byvirtue of the ability of the collar 64 to rotate about the carriage 60and the pivotal connection between the spooling block and the rotatablecollar 64 to pivot both sideways and fore and aft, to accommodatelateral and vertical changes of direction of the streamer as it leavesthe stern 18 of the vessel 10.

When it is desired to work on the streamer 28, eg to connect or remove adepth controlling “bird” or other device to it during deployment orrecovery, the carriage 60 is moved towards the other end 52 of the beam42 by the motors 62, and the telescopic arm 56 is extended, thuslowering the end 52 of the beam towards the streamer deck 22. As the end52 of the beam 42 is lowered, the curved guide 59 engages the streamer28, and pushes it down towards the streamer deck 22 until it reachesabout the same level as the top of stern rail 70 of the streamer deck.The stern rail 70 extends substantially the whole width of the streamerdeck 22, and is formed with a rearwardly-facing curved surface 72 whichsupports the lowered streamer 28. The surface 72, as well as each of theother curved surfaces which engage or are engaged by the streamers 28,is of sufficiently large radius to ensure that the minimum bendingradius of the streamer is not exceeded.

Before or after the streamer 28 is fully lowered, the winch 68 can becoupled to a suitable attachment point on it, and then reeled in alittle to take the strain off the portion of the streamer on the vessel,ie to the right of the attachment point to the hoist, as viewed in FIG.4. Then, once the streamer 28 is fully lowered and no longer undertension, it is very easy to work on, being typically at waist heightabove the streamer deck 22. If desired, a bench or the like can beprovided in the working area on the streamer deck 22, to make such workeven easier.

The winch 68 is also used to raise and bring onto the streamer deck 22auxiliary equipment associated with the streamer 28, eg tailbuoys,acoustic positioning equipment and the like.

The streamer handling apparatus 40 is thus particularly ergonomicallyadvantageous. In its normal (or towing) condition, it is mostly disposedabove head height above the streamer deck 22, leaving the deckrelatively clear for personnel to move about. But in its loweredcondition, it provides particularly good access to the streamer 28passing through it.

Many modifications can be made to the streamer handling apparatus 40.For example, the hydraulic motors 62 can be replaced by electric motors,while the telescopic arm 56 can be replaced by any other support devicecapable of moving the end 52 of the beam 42 up and down between itsraised and lowered positions. And the spooling blocks 34 can be replacedby any other suitable fairlead or pulley device.

Additionally, if there is no upper deck above the streamer deck 22, themounting means for mounting the beam 42 above the streamer deck cancomprise a suitably shaped frame supported on the streamer deck itself.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A seismic survey vessel having a stern,comprising: a displacement monohull; a streamer deck; powered drumapparatus and cooperating guide apparatus adapted for deploying andrecovering seismic streamers over said stern of said vessel via saidstreamer deck, said powered drum apparatus being mounted substantiallyamidships in said vessel, said streamer deck at said stern of saidvessel projecting laterally outwardly from the monohull of said vesselon both sides of a center line of the vessel and being wider than theremainder of said vessel, said cooperating guide apparatus including oneor more guide devices distributed across substantially the whole widthof said streamer deck for facilitating a lateral spreading of theseismic streamers.
 2. The seismic survey vessel of claim 1, wherein saidstreamer deck of said vessel is an upper deck at a particular height,said particular height being chosen such that, when the vessel is dockedat a dock, the deck of said vessel is above said dock.
 3. The seismicsurvey vessel of claim 1, wherein a width of said streamer deckdecreases progressively from the stern of said vessel until said widthis substantially equal to a width of said vessel amidships.
 4. Theseismic survey vessel of claim 1 wherein at least a part of said powereddrum apparatus is mounted in said vessel at a level which is lower thanthe level of said streamer deck.
 5. The seismic survey vessel of claim1, further comprising: a further deck below said streamer deck at saidstern of said vessel, said further deck including means for deployingand recovering seismic sources over the stern of said vessel.
 6. Theseismic survey vessel of claim 1, wherein each of said one or more guidedevices comprises a streamer handling apparatus which is associated withone of said seismic streamers, said streamer handling apparatuscomprising: a beam; means for mounting said beam above normal headheight above the streamer deck of said vessel such that the beam extendsgenerally longitudinally of the vessel, said means for mountingincluding a pivotal connection to the sternmost end of said beampermitting said end to pivot about a horizontal axis extending generallytransversely of the beam; extendable support means for lowering andraising the other end of the beam towards and away from the streamerdeck; and a respective one of said guide devices, each said guide deviceincluding a pulley apparatus secured to and movable along the beam. 7.The seismic survey vessel of claim 6, wherein said pulley apparatusincludes a pulley device which is pivotable about an axis that extendsgenerally transversely of the beam.
 8. The seismic survey vessel ofclaim 6, wherein said pulley apparatus comprises a carriage apparatusmovable along said beam, a pulley device, and means for suspending thepulley device beneath said carriage apparatus.
 9. The seismic surveyvessel of claim 8, wherein said means for suspending comprises a collarwhich is rotatably supported on the carriage apparatus and which isrotatable about a longitudinal axis of said beam.
 10. The seismic surveyvessel of claim 9 wherein said carriage apparatus includes at least onehydraulic motor arranged to move the carriage apparatus in bothdirections along said beam.
 11. The seismic survey vessel of claim 10,wherein the sternmost end of said carriage apparatus includes a winchapparatus.
 12. The seismic survey vessel of claim 11, wherein saidpulley apparatus includes a pulley device which is pivotable about ahorizontal axis that extends generally transversely of the beam.
 13. Theseismic survey vessel of claim 6 wherein said extendable support meanscomprises a telescopic member which is pivotally connected at one end tosaid means for mounting and at the other end to said other end of thebeam.
 14. The seismic survey vessel of claim 13, wherein said extendablesupport means is hydraulically operable.
 15. The seismic survey vesselof claim 6, further comprising: a downwardly projecting guidanceapparatus which is positioned at said other end of said beam and whichengages and pushes down said streamer when said other end of the beam islowered.
 16. The seismic survey vessel of claim 6, wherein said vesselincludes a further deck above said streamer deck, said means formounting being secured to an underside of said further deck.
 17. Aseismic survey vessel having a stern, comprising: a displacementmonohull; one or more powered drums; a streamer deck located adjacentsaid stern of said vessel; and one or more corresponding cooperatingguides adapted for deploying and recovering a corresponding one or moreseismic streamers via said streamer deck over said stern of said vessel,said one or more powered drums being distributed across the width ofsaid vessel, said streamer deck at said stern of said vessel projectinglaterally outwardly from the monohull of said vessel on both sides of acenter line of the vessel and being wider than the remainder of saidvessel, said one or more guides including one or more correspondingguide devices distributed across substantially the whole width of saidstreamer deck for facilitating a lateral spreading of the seismicstreamers, said streamer deck of said vessel being an upper deck at aparticular height, said particular height being chosen such that, whenthe vessel is docked at a dock, the deck of said vessel is above saiddock, a width of said streamer deck decreasing progressively from thestern of said vessel until said width is substantially equal to a widthof said vessel amidships.
 18. A seismic survey vessel having a stern,comprising: a displacement monohull; a streamer deck, said streamer deckat said stern of said vessel projecting laterally outwardly from themonohull of said vessel on both sides of a center line of the vessel andbeing wider than the remainder of said vessel; one or more powereddrums, said one or more powered drums being distributed across a widthof said vessel, at least a part of said one or more powered drums beingmounted in said vessel at a level which is lower than the level of saidstreamer deck; a further deck below said streamer deck at said stern ofsaid vessel, said further deck including means for deploying andrecovering seismic sources over the stern of said vessel; and one ormore corresponding cooperating guides on said streamer deck adapted fordeploying and recovering a corresponding one or more seismic streamersvia said streamer deck over said stern of said vessel, said one or moreguides including one or more corresponding guide devices distributedacross substantially the whole width of said streamer deck forfacilitating a lateral spreading of the seismic streamers.